The invention relates to a gas turbine operated with premix burners and the operation of a premix burner.
Premix burners are widely used in the operation of firing systems and, in particular, gas turbines. Such premix burners are known, for example from EP 321 809, EP 780 629, WO 93/17279, or EP 945 677. These burners are suitable for operation with liquid as well as gaseous fuels. A mutual characteristic of the burners disclosed in the cited documents is that combustion air is mixed in an inner chamber of the burner with a fuel as much as possible. The combustion air is frequently fed into the inner chamber of the burner tangentially. The eddy current induced in this way bursts open when exiting the burner opening. This so-called xe2x80x9cvortex breakdownxe2x80x9d generates a recirculation zone where a flame can be stabilized in a purely aerodynamic manner without attaching thermally sensitive, mechanical flame holders. By premixing air and fuel, these burners can be operated principally while avoiding stoichiometric zones, whichxe2x80x94at least during operation with gasxe2x80x94easily permits operation with very low emission values without any further measures.
When using liquid fuels, the premixing is relatively incomplete, so that in this case, a diffusion flame is actually present. Such burners are also referred to as premix burners below, since this term has become established for the cited burner types in the expert community. Premix burners operated with liquid fuel are frequently also operated additionally with steam or water injection so that relatively low emission values can be achieved, in particular with respect to nitrogen oxide emissions (these are values of less than 50 ppm).
An inert medium is introduced into the flame, which reduces both the flame temperature and therefore the nitrogen oxide production. When used in gas turbine combustors and with an injection of water, the injected quantity of water is approximately in the same magnitude as the quantity of fuel; the ratio of the quantity of water to fuel is roughly in the range from 0.7 to 2 in order to achieve a corresponding nitrogen oxide reduction. The corresponding operating concepts are adjusted in measuring series, and the quantity of water or, respectively, the ratio of the quantity of water to fuel, is determined depending on the operating conditions of the machine. With such an operation, the cited quantities of water naturally result in a high consumption of correspondingly prepared, demineralized water.
In the same way, for example, in a gas turbine equipped with premix burners, emission values that are clearly below those of gas turbines with standard burners with a so-called dry operation, i.e., without targeted introduction of water for nitrogen oxide reduction, can also be achieved during operation without water injection. Especially in arid regions, where, for example, no infrastructure for gas is present, or oil is especially readily available, gas turbines with premix burners, for example, so-called EV or AEV burners are operated in xe2x80x9cdryxe2x80x9d oil operation, i.e., there is no nitrogen oxide reduction by injection of water.
The statements made for oil naturally also apply for operation with other liquid fuels.
The burners are generally also operated dry during premix operation with gas, whereby the dry operation of the burners within the framework of the present application should be understood to mean that no water or steam is introduced into the combustion zone in large quantities in a targeted manner for nitrogen oxide reduction.
In practice, it has been found that in the large operating range, whereby in addition to the greatly varying parameters of temperature and pressure of the combustion air, primarily the thermal power turnover of a burner and the impulse ratio of fuel and air, as well as the ratio of the axial impulse to the rotating impulse of the burner flow must be mentioned, combustion instabilities occur that may result in undesired combustion pulsations. The position of the flame also changes. If the flame is stabilized too close to the burner mouth, this may result in temperature increases and an associated reduction in life span of the burner.
A method of operating a premix burner according to the invention includes introducing, in a targeted manner, a quantity of water depending on a command variable which is calculated from at least one measured operating parameter. In particular, this has the effect of varying the axial impulse of the burner flow in a targeted manner and independently from other operating parameters, such as the fuel quantity, and in this manner enabling independent shifting of the location of the flame stabilization within certain limits.
Within the framework of this disclosure of the invention, the term command variable means a dimension, depending on which the quantity of water that is introduced is adjusted.
In a preferred variation of the process, the burner is operated dry, i.e., without water injection for nitrogen oxide reduction. The introduced quantity of water is kept as low as possible, if at all possible at less than 20% of the quantity of the fuel. Preferably, it is attempted to limit the quantity of water to less than 10%, and even more preferably, less than 5%, of the quantity of the fuel.
During xe2x80x9cwetxe2x80x9d operation of a premix burner according to the method of the invention, i.e., during operation with water injection for reducing the nitrogen oxide, the process-specific variation of the quantity of the water is analogously kept in an advantageous manner to less than 20% of the fuel quantity. Preferably, it is attempted to limit the process-specific variation of the quantity of water to less than 10%, and even more preferably, less than 5%, of the quantity of the fuel.
This method can be used advantageously for operating the burner with liquid fuel.
In a preferred embodiment of the invention, the combustion pulsations present at least one measured value or parameter. As a command variable, for example, a summation value can be formed from this, or an amplitude peak of a frequency spectrum of the pulsations can be used as a command variable. Furthermore, one or more frequency ranges of the spectrum of the combustion pulsations also can be used to form the command variable.
In another preferred variation, at least one temperature is measured, in particular a material temperature of the premix burner, in order to form a command variable. In the same way, a signal of an optical sensor can be used. This would make it possible, for example, to obtain an indication of the flame position, which would be used as a command variable.
The water quantity adjustment introduced into the burner advantageously can be operated in a closed control circuit in order to adjust a command variable to a desired value, maintain it within a desired interval, or limit it to an upper or lower limit.
The quantity of water can be added via nozzles of the burner. It is also possible to mix the quantity of water prior to the injection of a liquid fuel into the combustor with the liquid fuel and in this way to operate the burner with a water-in-fuel emulsion.
The quantity of water either can be easily controlled simply depending on the command variable. On the other hand, the quantity of water also can be operated in a closed control circuit with the command variable as a regulating variable in order to keep, for example, pulsations or material temperatures below an upper limit or in order to achieve a desired value of the flame position.